The structural and operational simplicity of ion-specific electrodes makes them attractive for the determination of certain electrolytes and pH of the serum from critically ill patients. Further understanding of the time response characteristics of these electrodes would benefit this application. We have constructed a unique flow-through pH electrode and determined its time response characteristics using a programmable calculator system for data handling. We found that the sum of 3 exponentials was necessary to fit the time response curves. The time constants were strongly influenced by current flow through the electrode. The longer time constants represented 4 to 8 percent of the response and had a value of 4 to 12 minutes. A differential conductometric apparatus for the measurement of small amounts of CO2 has been constructed. This is an improved version of the continuous flow method developed by Steele and Maffly. This work is a first step in the development of a batch method for total serum CO2 based on this differential conductance principle.